1. Field of the Invention
This invention relates to polymeric compositions and the use of many of these compositions in electrophotographic elements and processes. More specifically, this invention involves random copolymers, many of which are photoconductive and, thus, suitable for use in electrophotographic imaging members and processes. The spacial constraint and relative conformation of the functional groups of the two principal components of these compositions apparently favors a charge transfer interaction between them.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well known. The best known of the commercial processes, more commonly known as xerography, involves forming a latent electrostatic image on an imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging layer in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored electroscopic material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface which retain the electrostatic charge and thus form a visible powder image.
The developed image can then be read or permanently affixed to the photoconductor where the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder-type photoconductive films (e.g. ZnO) where the photoconductive imaging layer is also an integral part of the finished copy.
In so-called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films, and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above "plain paper" copying system, the materials used in the photoconductive layer should preferably be capable of rapid switching from insulative to conductive to insulative state in order to permit cyclic use of the imaging surface. The failure of a material to return to its relatively insulative state prior to the succeeding charging sequence will result in a decrease in the maximum charge acceptance of the photoconductor. This phenomenon, commonly referred to in the art as "fatigue", has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling system include anthracene, sulfur, selenium and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of its superior photosensitivity.
In addition to anthracene, other organic photoconductive materials, most notably, poly(N-vinylcarbazole), have been the focus of increasing interest in electrophotography. Most organic photoconductive materials, includng poly(N-vinylcarbazole), lack the inherent photosensitivity to be competitive with selenium. This need for the enhancement of the photoresponse characteristics of organic photoconductors thus led to the formulation of these organic materials with other compounds, commonly referred to as "activators". Poly(vinylcarbazoles), for example, when sensitized with 2,4,7-trinitro-9-fluorenone exhibit good photoresponse and discharge characteristics and, (depending upon the polarity of the surface charge), low dark decay; U.S. Pat. No. 3,484,237. Other organic resins, traditionally considered nonphotoconductive can also be sensitized with certain activators, such as Lewis Acids, thus forming charge transfer complexes which are photoresponsive in the visible band of the spectrum, U.S. Pat. Nos. 3,408,181; 3,408,182; 3,408,183; 3,408,184; 3,408,185; 3,408,186; 3,408,187; 3,408,188; 3,408,189; and 3,408,190. With respect to both the photoconductive and nonphotoconductive resins, the degree of sensitization is generally concentration dependent; the higher the loadings of activators, the greater the photoresponse.
The concentration of activator capable of formulation with the above materials, however, is finite; generally being limited to less than 10 weight percent of the composition. Ordinarily, the addition of high loadings of activator to many of the above materials will lead to impairment of mechanical and/or the photoconductive properties of the sensitized composition. In most instances, the excessive addition of activators to both the photoconductive and nonphotoconductive materials of the types disclosed in the above patents will result in crystallization of these activators, thus impairing the mechanical strength and other physical properties of the resultant photo conductive composition. Still yet other sensitizers, when present in relatively low concentration can result in over sensitization of the composition in that the photocurrent generated upon exposure will persist long after illumination ceases, BUL. CHEM. SOC. of JAP. 39: 1660 - 1670 (1966). This phenomenon prevents the further use of such materials for preparation of successive electrostatic reproductions until such persistent conductivity is dissipated in the previously illuminated areas of the photoconductor. The dissipation of persistent photocurrents generally takes an extended period of time and/or thermal erasure, thus making these oversensitized compositions generally unsatisfactory for rapid cycling electrostatographic imaging systems.
As an alternative to the more traditional type of sensitization discussed above, Inami and Morimoto have proposed preparation of "intramolecular" charge transfer complexes wherein the electron donor and electron acceptor functions are located along a common vinyl backbone, U.S. Pat. No. 3,418,116. The materials of principal interest disclosed in the above patent are the nitrated vinyl polymers of polyacenaphthylene, poly-9-vinylcarbazole and poly-1-vinylnaphthalene. More recently, Podhajny has proposed his own "intramolecular" type charge transfer complex system wherein the electron donor and electron acceptor functions are contributed by 3,6-diphenyl-vinylcarbazole and 3,6-dinitrovinylcarbazole, respectively; U.S. Pat. No. 3,697,264. A more in depth treatment of this type of charge transfer complex system is offered by Breen and Keller, J. Am. Chem. Soc. 90, 1935, (1968). It is thought that the spacial constraint placed upon the electron donor and electron acceptor functions enhances the probability of charge transfer interaction. In addition, certain conformational and steric requirements must also be satisfied in order to facilitate efficient overlap of donor and acceptor electron orbitals required of this type of charge transfer interaction.
It is, thus, the object of this invention to provide polymeric compositions wherein the structural units thereof are from at least two vinyl monomers, one having an electron donor and a second having an electron acceptor function. More specifically, the principal object of this invention is to provide a photoconductive composition having an electron donor and an electron acceptor function.
It is another object of this invention to provide a photoconductive composition wherein the electron donor and electron acceptor functions are arranged along a common polymeric backbone.
It is yet another object of this invention to provide a photoconductive composition wherein the electron donor and electron acceptor functions are arranged along a common polymeric backbone in such a fashion as to favor "intramolecular" charge transfer complex formation.